Electric dynamic sea cables are electric power cables that may be connected to an off-shore installation. Off-shore installations may include floating platforms, platforms standing on the sea bottom, as well as floating vessels. Platforms are, for example, platforms by means of which oil and/or gas is exploited from sub-sea wells.
The electric power cables will typically be used to transmit electric power of medium or high voltage. In this description and the following claims “medium voltages” refers to voltages from about 1 kV up to about 40 kV, and “high voltages” refers to voltages from about 40 kV up to 800 kV or even above that figure.
A dynamic sea cable may be freely extending in the sea water between certain fixing points, for example the platform and the sea bottom and will therefore be moving with the motion of the sea, including sea currents and wind-induced motions. It is to be understood that part of an dynamic electric power sea cable can be located above the water level at the connection point to, for example, the platform or vessel.
A medium or high voltage three phase sea cable comprising three parallel conductors, each conductor surrounded with insulation and an outer protective sheath, behaves stiff only when bent a little. The protective sheath is usually made of metal and is also called a water barrier layer. Radially outside the outer protective sheath and also a part of the cable core a polymer layer could be arranged as protection. The insulation is, for example, polymer insulation, such as cross-linked polyethylene, or oil and paper insulation. “Cable core” will in the following description and claims refer to a conductor surrounded by insulation, the protective sheath and optional polymer layer.
All materials, but specifically the metals, in such a cable are exposed to mechanical fatigue due to the movements described above. Certain materials present lower fatigue strength and will therefore possibly crack if subjected to a certain degree of fatigue strain. Especially the water barrier layer, which usually is made of metal such as lead, copper or steel, is sensitive to fatigue and will eventually crack after too many or too large movements. This might lead to water ingress into the cable insulation which in turn might lead to a decrease in the electric properties of named insulation. An electric dynamic sea cable should therefore have a design that diminishes the forces acting on the internal parts of the cable, such as the water barrier layer.